Liquid alkyl aryl sulfones have proven useful as dielectric media, heat transfer fluids, plasticizers and functional fluids used in hydraulic systems, braking systems, servomechanisms and other devices. The individual isomers of certain alkyl aryl sulfones, however, are solids at room temperature, and are undesirable for fluid applications. Moreover, alkyl aryl sulfones containing impurities often interfere with and impede applications in which such sulfones are used. For example, chlorinated impurities in heat transfer fluids cause corrosion to metal parts in which the fluids are contained and circulated. In addition, condensed aromatic impurities cause limited transparency of the fluid in applications where optical clarity is desirable.
Alkyl aryl sulfones have been prepared by Truce and Vriegen, 75 J. Amer. Chem. Soc. 5032 (1953), by reacting alkyl sulfonyl chloride with aromatic hydrocarbons in the presence of aluminum chloride, ferric chloride or other Friedel Crafts catalysts. Typically, however, when alkyl groups are present on an aromatic hydrocarbon, extensive chlorination occurs in the presence of the alkyl sulfonyl chlorides and Friedel Crafts catalysts. For example, a 70% yield of 2,5-dimethylchlorobenzene is obtained from the reaction of p-xylene with methane sulfonyl chloride and aluminum chloride. See Hyatt and White, Synthesis 214-17 (1984).
To avoid side reactions and tar formation, various reagents, such as methane sulfonic anhydride or alkyl sulfonyl fluorides, have been used. See, e.g., Gilbert, 28 J. Org. Chem. 1945 (1963); Hyatt and White, supra. Alkyl aryl sulfones can also be made by reacting aromatic sulfonic acid salts with alkylating agents, such as methyl iodide or dimethyl sulfate. See, e.g., Oxley, et al., J. Chem. Soc. 763 (1946).
These methods have the disadvantage of using expensive chemicals, having long reaction times, failing to work with certain substituted aromatic hydrocarbons, yielding products that are solids or that crystallize over time, or yielding products that are difficult to purify. For example, the method using methane sulfonic anhydrides reacted with orthoxylene generally produces low yields and/or tar formation. Further, the reaction of methane sulfonyl chloride with hydrocarbons, such as toluene and xylene using ferric chloride as a catalyst, typically results in yields below 50% with dark-colored products, even after washing to remove the ferric chloride, and high boiling, chlorinated by-products, difficult to remove from the desired methyl aryl sulfone.
In view of the serious deficiencies and inefficiencies of the prior art, it is desirable to produce liquid mixtures of alkyl aryl sulfones and to have a simple method of preparation that is effective with all aryl positional isomers. While there are a number of methods known in the art for the preparation of diaryl sulfone using phosphorus compounds, see, e.g., U.S. Pat. No. 3,579,590 of Davis; U.S. Pat. No. 3,125,604 of Robbins, none of these methods suggest the preparation of a liquid mixture of positional isomers of alkyl aryl sulfones.